Compound friction vacuum pump

ABSTRACT

A friction vacuum pump ( 1 ) has at least one turbomolecular pump stage ( 6, 7 ) with a molecular pump stage ( 12, 13, 15 ) that is subsequently connected on the pressure side ( 3 ), and with a transition stage ( 23 ) mounted between the turbomolecular pump stage and the molecular pump stage. In order to improve the transition from the turbomolecular zone to the molecular zone, the transition stage has a flow section that is continuously tapered in the tangential by limiting surfaces ( 27, 28 ).

BACKGROUND OF THE INVENTION

The present invention relates to a friction vacuum pump comprising atleast one turbomolecular pump stage, with a molecular pump stage that issubsequently connected on the pressure side, and with a transition stagemounted between the turbomolecular pump stage and the molecular pumpstage.

In turbomolecular pumps with molecular pump stages, commonly designed asscrew pump stages, subsequently connected on the pressure side, alsocalled compound pumps, there exists the problem of a changing flowcharacteristic in the pumped gases in the transition area from molecular(at pressures below 10⁻³ mbar) to laminar (from about 10⁻² mbarupwards). As the pumped gas changes from the turbomolecular stage intothe screw stage, the pumped gas needs to be deflected from a primarilytangential direction of flow to a primarily axial direction of flow. Theradial dimension of the flow channel is tapered considerably. Across avery short distance, a large change in the axial cross section of thepump chamber needs to be implemented. Known embodiments of thistransition area have the disadvantage of incurring losses in the flow.These impair to a considerable extent the pumping capacity of the pump.

From DE 297 17 079 a friction vacuum pump having the aforementionedcharacteristics is known. Part of the transition stage is a centrifugalpump formed by ridges on the rotor side extending substantially in theradial direction. This solution does in fact have the effect ofdeflecting the gases into the screw stage; however, its pumping effectis limited. Moreover, the known solution requires that the diameter ofthe screw pump stage be greater than the diameter of the turbomolecularstage. For this reason it is not usable in high pumping capacityfriction pumps, since the diameter of the rotor in the molecular pumpstage is subject to restrictions in size owing to the high centrifugalforces. Finally as to the arrangement of the ridges on the rotor side itholds that their manufacture is involved and that they are notuncritical as to material tensions.

The content of U.S. Pat. No. 6,168,374, moreover, belongs to thestate-of-the-art. From this it is known to provide between theturbomolecular pump stage and the molecular pump stage that issubsequently connected, a filling stage which is equipped with wings.Also this solution is difficult to manufacture. Moreover, there resultduring operation, high mechanical tensions in the area of the wingsbases.

It is the task of the present invention to create a vacuum engineeringwise optimised transition of the turbomolecular range to the molecularrange without suffering the disadvantages detailed.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, the transitionstage is part of the stator and said transition stage has a flow sectionextending substantially in the tangential direction which is taperedcontinuously in the direction of the flow.

By shifting the transition stage into the stator, the objective isattained in that its design is free of material-related problems whichwould be observed when arranging the transition stage on the side of therotor due to the occurring centrifugal forces.

The present solution also takes in to account that the velocity of theflowing gases in the transition range is significantly greater in thetangential direction compared to the axial direction (a factor between10 and 30). For the avoidance of sudden changes in the flow section, itis for this reckon expedient to implement a tapered section extendingsubstantially in the tangential direction resulting in a low gradienttaper. The gradient depends on the number of blades in the transitionstage as well as on the ratio between blade length and diameter of thedownstream screw stage. The number of blades in the transition stage isdetermined on the basis of the same criteria which apply to the upstreamturbomolecular stages.

Thus an increase in pumping capacity is attained. Flow losses are thusreduced.

If the flow apertures designed in accordance with the present inventionare accommodated in a stator ring disk, then said apertures can bemanufactured in a simple manner by milling.

In the milling process, the cost-effective method of “spot facing” maybe employed, specifically with a cylindrical tool, provided the bladeswhich limit the flow section do not overlap. Should the blades overlap,manufacture can be effected by means of a milling cutter having on itsface side an increased diameter.

Further advantages and details of the present invention shall beexplained with reference to the examples of embodiments depictedschematically in drawing FIGS. 1 to 7.

Still further advantages of the present invention will become apparentto those of ordinary skill in the art upon reading and understanding thefollowing detailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating a preferred embodiment and are notto be construed as limiting the invention.

FIG. 1 is a sectional view through a compound pump in accordance withthe present invention,

FIG. 2 is a perspective view of a stator half ring designed inaccordance with the present invention,

FIG. 3 is a top view on to a section of a stator half ring depicted byway of a developed view,

FIG. 4 is a sectional view through a the stator half ring in accordancewith FIG. 3,

FIGS. 5 and 6 are top views on to further embodiments (developed views),and

FIG. 7 is a sectional view through a the stator half ring in accordancewith FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the example of an embodiment in accordance with drawing FIG. 1 thepump itself is designated as 1, its inlet as 2 its exhaust as 3. Thehousing of the pump 1 comprises the two sections 4 and 5.

The housing section 4 encompasses the stator 6 and the rotor 7 of theturbomolecular pump stage. The stator 6 comprises schematicallyindicated blade half rings 8 as well as spacing rings 9 which togetherform a self-centering stator pack. The rotor 7 is equipped with therotor blades 10. Only the stator half rings, the blades of whichtogether with the last row of rotor blades 10 on the pressure side formthe last turbomolecular pump stage on the pressure side, are depictedwith more detail and are designated as 23. Depicted in drawing FIG. 2 isa perspective view of one of these stator half rings 23.

The housing section 4 also encompasses the stator 15 and the rotor 12 ofthe screw or molecular pump stage, the pump chamber are defined by apumping slit is designated as 13. The thread 14 of this stage may bearranged on the stator or the rotor side. In the instance of thedepicted example of an embodiment its is arranged on the stator side andpart of a stator sleeve 15, fitted independently of housing section 4.The rotor 7 of the turbomolecular pump stage 7, 8 and the rotor 12 ofthe screw pump stage 11, 12 are parts of a jointly rotating system 7,12. The rotor 12 of the screw pump stage forms the end of this system onthe pressure side and may be designed as a disk or be bell shaped (asdepicted in FIG. 1).

The housing section 5 encompasses the drive motor 16, the stator ofwhich is designated as 17 and its rotor is designated as 18. The housingsection 5 is part of a chassis 19 with an internal chamber in which thedrive motor 16 and further components are located. Also accommodated inthe chassis 19 is the shaft 21 of the compound pump said shaft carryingthe rotors 7 and 12. Only the upper bearing 22 is visible. It is amechanical bearing, which may also be replaced by a magnetic bearing.Moreover, the chassis 19 is the carrier for all further components ofthe pump 1.

The stator half ring 23 depicted schematically in drawing FIG. 2consists of a half ring disk 24 with a multitude of flow apertures 25spread along its circumference. These are formed by blade sections 26which extend substantially radially and are preferably manufactured bymilling. The flow apertures 25 are designed in such a manner that there,in all, results a flow section extending substantially in the tangentialdirection and continuously tapered in the direction of the flow. This isattained by the length of the blade sections 26 (their radial extension)being greater on the intake side (Is) compared to the pressure side(Id), i.e. that the distance of the lateral limiting surfaces 27, 28 ofthe flow apertures 25 decreases with respect to the direction of theflow.

Drawing FIGS. 2, 3 and 4, depict embodiments of a stator half ring 23 inwhich the blade sections 26 do not overlap. They allow viewing on to thehalf ring disk 23 in the axial direction (indicated by the arrow 38 indrawing FIG. 4). In the instance of stator half rings of this kind, theblade sections may be manufactured through “spot facing” with acylindrically designed tool 29 (see FIG. 4).

In the embodiments in accordance with FIGS. 5 to 7, the blade sections26 overlap which respect to each other. Also these embodiments allowmanufacturing of the stator half rings by milling. Here, the tool 29 hasan increased diameter at its face side (see FIG. 7).

In drawing FIGS. 3, 5 and 6, the pumping slot 13 of the screw stage 12,15 located downstream of the turbo stages is indicated by dashed linesin each instance. In these embodiments the pumping slots 13 differ indiameter d₁, d₂, d₃. The flow apertures 25 designed in accordance withthe present invention allow an adaptation to different diameters for thepumping slot 13. This may be effected in that the position of thelimiting surfaces 27, 28 and specifically their inclination with respectto the respective tangents is selected such that the pumping slot 13 islocated approximately in the middle of the radial extensions Id of theblade sections 26 on their pressure side.

The invention has been described with reference to the preferredembodiment. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the invention be construed as includingall such modifications and alterations insofar as they come within thescope of the appended claims or the equivalents thereof.

1. A friction vacuum pump comprising: at least one turbomolecular pumpstage; a molecular pump stage connected on a pressure side of theturbomolecular stage; and a transition stage mounted between theturbomolecular pump stage and the molecular pump stage, the transitionstage being formed by the last row of stator blades arranged on thepressure side, the transition stage having a flow section that iscontinuously tapered in a tangential direction.
 2. The pump according toclaim 1, wherein the stator blades are parts of a stator ring diskhaving two half rings and each half ring having limiting flow aperturescontinuously tapered in the tangential direction.
 3. The pump accordingto claim 2, wherein lateral limiting surfaces of blades limit the flowapertures a distance between the lateral limiting surfaces decreasing inthe direction of the flow.
 4. The pump according to claim 3, wherein thedecrease in the distance of the limiting surfaces is so designed that apumping slot of the molecular pump stage downstream of theturbomolecular stage is located approximately at the middle of radialwidths of the blades on their pressure side.
 5. The pump according toclaim 1, wherein the blades of the transition stage are manufactured bymilling or casting.
 6. A vacuum pump comprising: a turbomolecular stagehaving a suction end and a pressure end; a transition stator ring at thepressure end, the stator ring having a plurality of angled slots whichdirect flow from a primarily tangential direction to a primarily axialdirection, the slots being larger at a turbomolecular stage side than atan output side; a screw pump stage connected with the output side of thetransition stator ring.
 7. A vacuum pump comprising: a turbomolecularstage having a suction end and a pressure end; a downstream pump stagedownstream from the turbomolecular stage; a transition stator ring atthe turbomolecular stage pressure end, the stator ring having aplurality of angled slots which direct flow from a primarily tangentialdirection to a primarily axial direction, the slots being defined onradial edges by limiting surfaces which converge from the turbomolecularstage toward the screw pump stage.